Hi Farheen!
You can follow the study schedule, in which case 72 will be assigned as your diagnostic, but my suggestion (and sorry if I wasn’t clear on this) would be to not follow a study schedule (which includes lots of extra practice), but rather just finish the book + take a diagnostic — for that diagnostic, you can use whatever test you’d like — 72 will work fine — another suggestion for the diagnostic is the June 07 Exam, which LSAC offers for free here —
https://www.lsac.org/docs/default-source/jd-docs/sampleptjune.pdf
A nice advantage of using the June 07 as your diagnostic is that you can access many free explanations for the problems — for example, you can find free video explanations of the Logic Games from that exam on my YouTube channel —
HTH clear things up and let me know if you need anything else — MK